As examples that necessitate accurately polishing a surface of a material, an optical disc substrate, a magnetic disc, a glass substrate for use in flat panel displays, a watch plate, a camera lens, a glass material used for various kinds of lenses for use in optical components, a crystal material such as filters, a substrate such as silicon wafer for use in semiconductor or the like, and an insulating film, a metal layer, a barrier layer and so on formed in the respective steps in the manufacture of a semiconductor device can be cited. Surfaces of the materials are required to polish with high precision. Accordingly, a polishing agent in which for instance silica, zirconium oxide, alumina and so on are used singularly or in a combination of at least two kinds thereof as polishing particles is generally used. As a form of the polishing agent, for instance, one in state of slurry in which polishing particles are dispersed in a liquid, one in which polishing particles are solidified together with a binder such as a resin or others, and one in which polishing particles are adhered and/or fixed on a surface of a base material such as fiber, resin, metal or the like by use of fine particles alone or together with a binder are generally used as the polishing agent.
In particular, since a silica based polishing slurry where fine particles of silica are used as polishing particles generate less scratch on a surface being polished, it is widely used as a precision polishing slurry of semiconductor integrated circuits. However, since the polishing speed thereof is slow, recently, a cerium oxide based polishing slurry containing cerium oxide, which is rapid in the polishing speed is gathering attention (Japanese Patent Application Laid-Open Nos. 2000-26840 and 2002-371267). However, there is a problem in that the cerium oxide based polishing slurry is much in the scratch relative to the silica based polishing slurry.
Cerium oxide used mainly in the cerium based polishing slurry can be produced by calcining a cerium salt at a high temperature followed by as needs arise pulverizing and classifying.
In the beginning, as a method of producing the cerium salt, for instance, firstly, a cerium compound such as an ore containing rare earths containing at least cerium (bastnaesite ore, heavy sand, parankerite and so on) or the like is subjected to a mineral processing (separation, acid leaching and so on) to remove other valuable matters and unnecessary veinstones, and thereby a rare earth concentrate (bastnaesite concentrate, monazite concentrate, Chinese complex concentrate and so on) is obtained. In the next place, the rare earth concentrate is chemically processed (alkali decomposition reaction, sulfuric acid decomposition reaction, hydroxide fractional precipitation and so on) to reduce insoluble components such as impurities or the like, further subjected to a solvent extraction, as needs arise, to reduce rare earths such as neodymium, and thereby a cerium-containing rare earth salt solution is obtained. To the cerium-containing rare earth salt solution, a precipitating agent (ammonium hydrogen carbonate, aqueous ammonia, sodium hydrogen carbonate, sodium carbonate, oxalic acid and so on) is added to generate a precipitate (rare earth carbonate, rare earth hydroxide, rare earth oxalate and so on), and thereby a cerium salt is obtained (the Japanese Patent Application Laid-Open No. 2002-371267).
Furthermore, since the cerium-containing rare earth salt solution costs much in transportation or storage thereof, in some cases, the rare earth salt solution is concentrated under heating, followed by being cooled and solidified to obtain rare earth chloride or the like. After transporting or storing, the rare earth chloride is dissolved with water or diluted acid again to be used as the cerium-containing rare earth solution.
Furthermore, as shown in the Japanese Patent Application Laid-Open No. 2002-371267, thus obtained cerium salt (rare earth carbonate, rare earth hydroxide, rare earth oxalate and so on), as needs arise, is further subjected to filtration, pulverization, chemical processing such as mineral acid processing or fluorination, dewatering, drying and so on.
Among cerium salts, as a producing method of cerium carbonate, various methods are known besides the above-mentioned producing method of rare earth carbonate (cerium salt). One of those generally used is a method where carbonate groups more than equivalent amount to rare earth ions are charged to obtain fine carbonate powder (Japanese Patent Application Laid-Open No. 53-095900). Furthermore, there are also a method where, in order to further purify, for instance to reduce insoluble components of alkaline earth metals, crude rare earth oxide is dissolved in a mineral acid aqueous solution, the rare earth metals are precipitated as oxalates in an acidic region where alkali earth metals are difficult to precipitate, and the oxalates are calcined to obtain oxides, further dissolved again in mineral acid such as hydrochloric acid, nitric acid or the like to precipitate as a carbonate; a method where rare earth elements and alkali earth elements are separated by means of an ion exchange method or a solvent extraction method; and a method where when cerium carbonate is generated from a cerium mineral acid salt aqueous solution the pH of the cerium mineral acid salt aqueous solution is kept in a definite range (Japanese Patent Application Laid-Open No. 07-144915) and so on.
Disclosure of Invention
However, when cerium oxide that is produced by processing at high temperatures the cerium salt obtained according to those methods is used as polishing particles in a cerium oxide based polishing slurry, it is difficult to reduce the scratch on a surface being polished.
The invention intends to provide a cerium based polishing slurry that can be used with reduced scratch in a field where a material surface is necessary to be precisely polished, in particular in semiconductors, liquid crystal displays, hard discs and so on, cerium oxide used therein as the polishing particles, a cerium salt that is a raw material used to produce the cerium oxide and a producing method thereof.
The inventors, after studying hard to reduce the scratch when the cerium based polishing slurry is used, found that the scratch can be reduced when an amount of fine particles of impurities and so on contained in cerium oxide particles used in the cerium based polishing slurry and cerium salt particles that are raw material of the cerium oxide particles is reduced, and came to the invention.
That is, the invention relates to (1) through (11) below.
(1) A cerium salt where, when 20 g of the cerium salt is dissolved in a mixed liquid of 12.5 g of 6N nitric acid and 12.5 g of a 30% hydrogen peroxide aqueous solution, a concentration of an insoluble component present in the solution is 5 ppm or less by mass ratio.
(2) The cerium salt according to the (1), wherein the insoluble component is a substance containing silicon.
(3) A producing method of a cerium salt where through steps of obtaining one kind or a plurality of kinds of cerium-containing intermediates from a cerium compound a precipitating agent is added to obtain a precipitate of cerium salt, wherein at least one step of separating and removing an insoluble component from the cerium-containing intermediate in a solution state is included.
(4) The producing method of a cerium salt according to the (3), including a step of adding a precipitating agent to the cerium-containing rare earth salt solution to obtain a precipitate of the cerium salt, wherein a precipitating agent from which an insoluble component is removed in advance is added to the cerium-containing rare earth salt solution to precipitate.
(5) The producing method of a cerium salt according to the (3) or (4), further including a step of obtaining a solution by mixing the cerium salt and 6N nitric acid, a step of separating and removing an insoluble component in the solution, and a step of adding after the removal a precipitating agent to precipitate a purified cerium salt.
(6) The producing method of a cerium salt according to any one of the (3) through (5), wherein the precipitating agent is in a solution state where the precipitating agent is dissolved in advance in a solvent to separate and remove the insoluble component.
(7) Cerium oxide obtained by subjecting the cerium salt obtained according to the (1) or (2) or the cerium salt obtained according to the producing method according to any one of the (3) through (6) to a high temperature process of 250° C. or more.
(8) The cerium oxide according to the (7), wherein when 20 g of the cerium oxide is dissolved in a mixed liquid of 12.5 g of 6N nitric acid and 12.5 g of a 30% hydrogen peroxide aqueous solution, a concentration of an insoluble component present in the solution is 10 ppm or less by mass ratio.
(9) Cerium oxide in which when 20 g of the cerium oxide is dissolved in a mixed liquid of 12.5 g of 6N nitric acid and 12.5 g of a 30% hydrogen peroxide aqueous solution, a concentration of an insoluble component present in the solution is 10 ppm or less by mass ratio.
(10) A cerium based polishing slurry that contains the cerium oxide according to any one of the (7) through (9).
(11) A cerium based polishing slurry in which when 20 g of the cerium based polishing slurry is dissolved in a mixed liquid of 12.5 g of 6N nitric acid and 12.5 g of a 30% hydrogen peroxide aqueous solution, a concentration of an insoluble component present in the solution is 10 ppm or less by mass ratio.